Simply put, another word for good, great, okay, fine, fantastic, etc. Only used by Stewie from Family Guy.

Actual Quotes from Family Guy:

Meg: I can't believe this is happening to me. I can never go back to school again.

Stewie: Oh yes, Meg. Yes, everything was going SWIMINGLY for you until this. Yes, this is the thing that will ruin your reputation. Not your years of grotesque appearance, or awkward social graces. Or that Felix Ungerish way you clear your sinuses. No, no, it's this. Do you hear your self talk? I might kill you tonight.